The mechanism of the immunotherapeutic effect of soluble tumor antigens to enhance host resistance was investigated in several models utilizing methycholanthrene-induced murine fibrosarcomas in C3H/HeJ mice. In a model to establish the effect of antigen therapy upon sineconcomitant immunity, animals underwent tumor resection 14 days after inoculation with 105 MCA-F cells, when the neoplasm was 1 cm. Either at 1 day after operation or at 14 days after operation, animals received one or two injections of 25 micrograms of isoelectrically purified tumor antigen (Fr15) extracted from homotypic neoplasms with 3M KCl. Animals that initially received Fr15 at 1 day after resection displayed decreased survival compared to hosts whose immunotherapy was postponed to 14 days. In the latter case host survival was prolonged. This shows that administration of Fr15 at a time when the host possesses concomitant immunity adversly affects the outcome of the animal. Therefore, it appears that to obtain maximal efficacy, one should delay antigen therapy until the decay of concomitant immunity. The effect of splenectomy at the time of tumor resection in combination with Fr15 therapy revealed that splenectomy by itself at the time of tumor resection did not alter the outcome. Splenectomy together with Fr15 therapy did not alter the tumor growth facilitation caused by early antigen therapy, suggesting that the therapy is not mediated solely by activation of splenic suppressor cells. The immunotherapeutic effect of MCA-F 3M KCl extract Fr15 prepared by isoelectric focusing was tested on sublines isolated from MCA-F, namely clones C3, G9 and E7. Animals inoculated subcutaneously with 104 cells of a subline and then treated with parental MCA-F Fr15 at weekly intervals displayed decreased growth of all clones compared with untreated individuals, demonstrating that shared TSTA can be harnessed for an immunotherapeutic effect. These studies will thus dissect major variables which determine the efficacy of soluble antigens as immunotherapeutic agents for neoplastic disease.